This project seeks an understanding of the photophysical processes leading to lethal or mutagenic damages induced in DNA by UV-light. Specifically, we study the behavior of the triplet purin and pyrimidine base molecules: their phosphorescence decay characteristics, their zero-field magnetic resonance spectroscopy and their intersystem crossing pathways. We are particularly interested by the role played by the C equals O group in the base molecules, their mechanism in enhancing spin-orbit coupling, and the extent hydrogen bondings render these mechanisms inoperative. For this purpose model carbonyl compounds are also studied.